ultralytics 8.0.31 updates and fixes (#857)

Co-authored-by: Yonghye Kwon <developer.0hye@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Kalen Michael <kalenmike@gmail.com>

ultralytics/yolo/data/dataset.py

@@ -132,7 +132,7 @@ class YOLODataset(BaseDataset):
             for lb in labels:
                 lb["segments"] = []
         if len_cls == 0:
-            raise ValueError(f"{self.prefix}All labels empty in {cache_path}, can not start training. {HELP_URL}")
+            raise ValueError(f"All labels empty in {cache_path}, can not start training without labels. {HELP_URL}")
         return labels
 
     # TODO: use hyp config to set all these augmentations